1. Field of the Invention
This invention relates generally to power plants and more particularly to a pressure fluid source and motor of the type including selective or simultaneous power and manual energy inputs.
2. Description of the Prior Art
In the past, hydraulic systems such as those used to control an implement system in an excavator have included selective pressure increasing ability to the boost power requirements of the system from a normal maximum operating pressure to an increased maximum allowable operating pressure. Prolonged operation of the system at an increased pressure, however, may be detrimental due to resulting increased stress on the system components. Variable displacement pumps in the system are usually biased to operate at a relatively high flow output. Thus, the combination of increased pressure at a relatively high flow is additionally detrimental to the system. For example, in an excavator, the operator may experience a force deficiency in attempting to actuate an implement for lifting a load. By selectively increasing the maximum allowable system pressure, the force deficiency may be overcome and the boosted or increased allowable pressure may be sufficient to lift the load. Realizing the force advantage, the operator may prefer not to return to operating at the normal maximum allowable system pressure. As a result, the implement circuit is permitted to continue to operate under increased pressure and, simultaneously, the pumps continue to operate at their normal high flow output. Prolonged operation at increased pressure and high flow will increase system operating stresses that will ultimately reduce the lifetime of the system.
Also, in the past, speed sensing circuits have been used in hydraulic control systems to respond to an overload imposed on the prime power source of an associated vehicle caused by an increase in system pressure. The speed sensing circuit responds to such an overload by automatically reducing the pump output or flow. A pressure sensing circuit has also been used to sense an increase in system pressure and responds to automatically stroke the pumps back to reduce their output.
Unfortunately, no system has provided the selective pressure control simultaneously interacting with a flow control.
In view of the above, it would be advantageous to provide a hydraulic system having selective simultaneous pressure and flow control which overcomes the problems associated with the prior art.